GENEVA (Reuters) - The presidents of Iran, Russia and Turkey - the main outside players in Syria’s long war - were meeting in Tehran on Friday to discuss the fate of the rebel-held province of Idlib as rival factions there prepared for what could be the conflict’s last great battle.

With Syrian government forces backed by Russian air power preparing an offensive against the insurgents in their last major stronghold, the United Nations has warned an assault could lead to a humanitarian catastrophe.

Beyond that concern, the three leaders will be seeking to safeguard their own interests after investing heavily militarily and diplomatically in Syria. Russia is looking to the summit as a means of clarifying the military situation in Idlib, deputy foreign minister Sergei Ryabkov said on Wednesday. Russian officials have also stated clearly that militants must be pushed out of Idlib. The most powerful jihadist group there is the Tahrir al-Sham alliance, spearheaded by al Qaeda’s former Syrian affiliate, previously known as the Nusra Front.

Turkey agreed with Russia and Iran last year to make Idlib a “de-escalation zone” of reduced conflict and sent in army units to erect a string of observation posts along the frontlines between the rebels and the army. They now act as a tripwire for any Syrian government assault, potentially triggering an escalation with Turkey if there is no political deal.